1. Field of the Invention
Technology described herein relates to manufacture of semiconductor devices. More specifically, methods are described of forming field effect transistors using group IV alloy materials.
2. Description of the Related Art
Germanium was one of the first materials used for CMOS transistors. Due to vast abundance of silicon compared to germanium, however, silicon has been the overwhelming semiconductor material of choice for CMOS manufacture. As device geometries decline according to Moore's Law, the size of transistor components poses challenges to engineers working to make devices that are smaller, faster, use less power, and generate less heat. For example, as the size of a transistor declines, the channel region of the transistor becomes smaller, and the electronic properties of the channel become less viable, with more resistivity and higher threshold voltages.
Carrier mobility is increased in the silicon channel area by using silicon-germanium stressors embedded in the source/drain areas, which enhances the intrinsic mobility of silicon. For future nodes, however, still higher mobility devices are needed.
Switching to higher mobility materials than silicon, such as germanium for pMOSFETs, has been suggested. However, the mobility of germanium is not superior to strained silicon, unless the germanium is also strained. It has been recently discovered that germanium tin (GeSn) grown on the source drain region has the requisite strain for making a superior germanium pMOSFET channel, which takes advantage of the germanium/GeSn lattice mismatch.
Conductivity across one or stacks of material structures is an important facet of CMOS formation. Overall conductivity is a function of carrier mobility, carrier concentration, and band alignment between materials. GeSn is attractive in these aspects. High carrier mobility layers will benefit more from increased carrier concentration than low mobility layers. Doping is one means to increase carrier concentration, however the methods of doping a GeSn layer have not been disclosed in the art. Thus, there is a continuing need for methods and apparatus to selectively form high mobility semiconductor devices as well as manipulate the related conductivity.